Bitcoin dominance is not decreasing and it's an obvious statistics that you can get by looking at both market cap and how much liquidity bitcoin has compared to the entire crypto ecosystem. Basically when bitcoin pumps it has a huge influence on the general market and that is part of the factors that determine the dominance of a particular currency over others.

The only thing that is arguably true is Bitcoin dominance in P2P tradings which I used the term arguably since from a point of view everyone focuses more on hodling than actual P2P.